The ownCloud thumbnail system generates previews of files for all
ownCloud apps that display files, such as Files and Gallery.

The following image shows some examples of previews of various file types.

By default, ownCloud can generate previews for the following filetypes:

Images files

Cover of MP3 files

Text documents

Note

Older versions of ownCloud also supported the preview generation
of other file types such as PDF, SVG or various office documents.
Due to security concerns those providers have been disabled by
default and are considered unsupported.
While those providers are still available, we discourage enabling
them, and they are not documented.

Under certain circumstances, for example if the server has limited
resources, you might want to consider disabling the generation of previews.
Note that if you do this all previews in all apps are disabled, including
the Gallery app, and will display generic icons instead of
thumbnails.

There are two configuration options for setting the maximum size (in pixels) of a preview.
These are preview_max_x which represents the x-axis and preview_max_y which represents the y-axis.
In config/config.sample.php, which you can see below, both options are set to to a default of 2048.

<?php
'preview_max_x' => 2048,
'preview_max_y' => 2048,

The following example would limit previews to a maximum size of 100 px × 100 px:

<?php
'preview_max_x' => 100,
'preview_max_y' => 100,

Note

If you want no limit applied for one or both of these values then set them to null.

If a lot of small pictures are stored on the ownCloud instance and the preview
system generates blurry previews, you might want to consider setting a maximum
scale factor. By default, pictures are upscaled to 10 times the original size:

<?php
'preview_max_scale_factor' => 10,

If you want to disable scaling at all, you can set the config value to ‘1’:

<?php
'preview_max_scale_factor' => 1,

If you want to disable the maximum scaling factor, you can set the config value
to ‘null’: